on a related subject (if the OP wants me to make another thread i can), does anybody know how painless suffocation by baking soda is?

i think it's related to gassing with alka-seltzer, but the last time i was at a friend's house we felt it necessary to put down one of her EXTREMELY sick fish (he was under a piece of driftwood with his tail down, shaking and he'd been that way for days) and we weren't sure how, so we googled methods and we found that you can put a whole lot of baking soda in water, mix, and drop the fish in and they'll die...

it seemed to pretty instantly take away his ability to swim, he floated to the top of the jar in seconds, and he stopped moving his body but his gills continued to move, which made me wonder if he wasn't in pain while alive? i'm not sure if there's any way to know what moment a fish stops feeling pain, but has anyone else used this method? what were your experiences?